Electronic devices or systems such as computers and printers may contain various grounds. A ground is a conducting connection used to establish and maintain the electrical potential of the earth or other grounding body on conductors connected to it.
For example, a computer printer may be grounded to the earth by a ground lead contained in a 3-wire power cord connecting the printer to an electrical outlet. Additionally, a computer printer may include a chassis ground wherein the printer's chassis is the grounding body.
An electronic device may also contain a logic or circuit ground which provides a reference ground potential to components such as digital logic circuitry. A logic ground generally uses the earth ground as the grounding body. However, the conductive path from the logic ground to the point of connection with the earth ground is often lengthy, resulting in a high inductance. Such high inductance, with current flow, creates an electromotive force in the circuitry which affects the ability of the conductive path to maintain a certain ground potential or to discharge certain charges to the earth.
Electronic devices such as computers and printers are often also exposed to electrostatic discharge. Electrostatic discharge is a dissipation of an electric charge often causing spurious sharp voltage pulses that can damage or interfere with the operation of electronic equipment. Such charges may originate from varied sources such as a user touching a keyboard or paper being fed into a printer.
To prevent the possibility of damage from electrostatic discharge, electronic devices may be shielded. Shielding may consist of an electroplate coating on the surrounding enclosure. However, such shielding is expensive and difficult to employ.
Alternatively, a separate conductive path with a low inductance to earth ground may be used. Such a path may be supplied by using electronic components such as a four-layer printed circuit board. In such a case, one or more layers of the board may be dedicated to provide a ground plane. However, additional board layers are also relatively expensive. In addition, it is desirable to steer the electrostatic discharge current away from the electronic components. Accordingly, a more desirable low inductance path would be through the chassis, which is connected to earth ground. Such a path is referred to as chassis ground.
When digital logic or other electronics performs under normal operation, it draws time-varying current through logic ground. When there is inductance between logic ground and earth ground, the logic ground becomes contaminated. Contamination refers to unnecessary high frequency signals superimposed on the desirable signal.
Additionally, electronic devices often create, and are affected by, electromagnetic interference. Such interference is undesirable contamination of a signal which tends to obscure the information content of the signal. To minimize electromagnetic interference, it is desirable to filter out the contamination. A clean ground must be used to filter out the contamination; if the contaminated ground were directly connected to the clean ground, the clean ground would become contaminated, and the filter would not function. So the contaminated ground, such as logic ground, must remain isolated from the clean ground, such as the chassis ground.
This invention overcomes the limitations of the prior art by providing an inexpensive protection circuit which conducts electrostatic discharge by way of a low inductance path to a ground capable of dissipating such signals, while isolating the "contaminated" ground, such as the logic ground, from a "clean" ground in the system, such as the chassis ground.